Metolachlor chemical structures

Chloroacetanilides are soil-applied herbicides used for pre- and early post-emergence control of annual grasses and broadleaf weeds in crops. Representative chloroacetanilide compounds, alachlor, acetochlor, and metolachlor, are extensively used worldwide. Other chloroacetanilides with limited usages include propachlor, bu-tachlor, metazachlor, pretilachlor, and thenylchlor. Public environmental concerns and government regulatory requirements continue to prompt the need for reliable methods to determine residues of these herbicides. There now exist a variety of analytical methods to determine residues of these compounds in crops, animal products, soil, and water. The chemical structures and major crops in which these compounds are used are summarized in Table 1. 

Figure 3.35 a Chemical structures of the four stereoisomers of metolachlor b 3D representation ofthe (aR, l S) isomer, showing the chiral axis and c the asymmetric imine hydrogenation step. 

Figure 1. Chemical structures of (a) alachlor, (b) acetochlor, and (c) metolachlor.

When it became clear that the two IS-enantiomers of metolachlor were responsible fijr most of the biological activity (see Fig. 1), there was the obvious challenge of finding a chemically and economically feasible production process for the active stereoisomers. Many methods allow the enantioselective synthesis of chiral molecules (that is the preferential formation of one enantiomer instead of the usual racemate). However, the selective preparation of S-metolachlor was a formidable task, due to the very special structure and properties of this molecule and also because of the extremely efficient production process for the racemic product as described above. During the course of the development efforts, the following minimal requirements evolved for a technically viable catalytic system ee S80%, substrate to catalyst ratio (s/c) >50 000 and turnover fi-equency (tof) >10 000 h" . 

Al ough alachlor is no longer used in the U.S., the three chemical compounds have very similar structural (Figure 1) and chemical properties. Alachlor degradataion data may be useful as a model for this chemical class. Caution must be used in interpolating these data however since the ESA metabolite of metolachlor is formed more slowly and at lower concentrations in soil (18). The objective of this study was to compare atrazine and alachlor sorption, mineralization, and degradation potential, processes that are major contributors to the environmental fate of pesticides, from surface soil to aquifer sediments in laboratoiy studies. In addition, ctegradation of alachlor was compared under aerobic and anaerobic conditions. 

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